Weakening-strengthening effect and micromechanisms of slip zone soil immersed in Ca²⁺-enriched water

Prolonged groundwater exposure weakens reservoir landslide slip zone soils, yet the specific role of calcium ion-rich groundwater remains unclear. To address this, slip zone soils from the Huangtupo landslide in the Three Gorges Reservoir area were immersed in deionized water and calcium ion-enriched solutions for 40 days, followed by reverse direct shear tests. Comprehensive analyses tracked particle size distribution, microstructure, leachate chemistry, and mineral phases over time. Results show deionized water significantly reduced shear strength, while calcium ions counteracted weakening by increasing peak friction angle and residual cohesion. Microstructurally, calcium ions suppressed particle breakage and promoted stable aggregation, unlike deionized water-induced fragmentation. Mechanistically, calcium ions modulate the diffuse double layer thickness and electric potential, shifting soil behavior from breakage-dominated weakening to aggregation-stabilized strengthening. This reveals calcium ions' critical regulatory role in slip zone mechanics, offering vital insights for reservoir landslide risk assessment and mitigation.